Bump xunit from 2.7.0 to 2.8.0

Bumps [xunit](https://github.com/xunit/xunit) from 2.7.0 to 2.8.0.
- [Commits](xunit/xunit@2.7.0...2.8.0)

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

src/firely-net-sdk-tests.props

@@ -23,7 +23,7 @@
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
 		<PackageReference Include="MSTest.TestFramework" Version="3.3.1" />
 		<PackageReference Include="MSTest.TestAdapter" Version="3.3.1" />
-		<PackageReference Include="xunit" Version="2.7.0" />
+		<PackageReference Include="xunit" Version="2.8.0" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
 			<PrivateAssets>all</PrivateAssets>
 			<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>